Chapter 4: The Tissue Level of Organization – Vocabulary Flashcards

What Is a Tissue?

• A tissue is a group of cells that usually have a common embryonic origin and function together to carry out specialized activities.

• There are $4$ basic types of tissues in the human body, categorized by structure and function.

The 4 Tissue Types

• Epithelial tissue

• Connective tissue

• Muscular tissue

• Nervous tissue

Cell Junctions- linking of cells (point of contact)

• Cells can be held together in several ways; these contacts between cells are called cell junctions.

• Types include:

  • Tight junctions

  • Adherens junctions

  • Desmosomes

  • Gap junctions

  • Hemidesmosomes

Tight Junctions-fused

• Adjacent plasma membranes are sealed together by strands of trans-membrane proteins.

• Create a barrier to prevent paracellular movement of substances between cells.

• Often located near the apical surface of epithelial layers.

Adhering Junctions (Adherens Junctions)- sticky glue like substance between two cells

• Use transmembrane glycoproteins (cadherins) connected to an intracellular plaque.

• Microfilament network (actin) attaches to the plaque inside the cell.

• Form an adhesion belt around cells to resist separation during contractile activity.

Desmosomes- cell membrane thickens, forming a thick plaque. Strongest binding between cells

• Button-like junctions that tie cells together at specific points.

• Use transmembrane glycoproteins (cadherins) linked to intermediate filaments (keratin) inside the cell.

• Provide strong, durable adhesion, especially in tissues subjected to mechanical stress.

Hemidesmosomes

• Attach epithelial cells to the basement membrane, not to another cell.

• Involve transmembrane glycoprotein (integrin) linking intermediate filaments to the basement membrane.

• Anchor epithelial layers to connective tissue.

Gap Junctions-opening between two cells, tunnel, channel connecting the cells together

• Channels formed by connexons (composed of connexins) that connect adjacent cell cytoplasm.

• Allow ions and small molecules to pass directly between cells, enabling rapid intercellular communication.

• Essential for synchronized activities in many tissues (e.g., heart, smooth muscle).

Summary: Cell Junctions

• Tight junctions: seal and create a barrier

• Adherens junctions: belt-like adhesion via actin filaments

• Desmosomes: strong adhesion via intermediate filaments

• Hemidesmosomes: anchor to basement membrane

• Gap junctions: intercellular channels for communication

Comparison Between Epithelial and Connective Tissues

• Epithelial tissue:

  • Contains many cells tightly packed together

  • Little extracellular matrix (ECM)

• Connective tissue:

  • Contains relatively few cells scattered in substantial ECM

  • Rich in extracellular fibers and ground substance

Epithelial Tissue

• General features: covers/lines

  • Cells are arranged in sheets and are densely packed

  • Many cell junctions are present

  • Cells attach to a basement membrane

  • Avascular (no blood vessels) but innervated (nerve supply present)

  • Mitosis occurs frequently

Surfaces of Epithelial Cells and the Basement Membrane

• Surfaces:

  • Apical (free) surface

  • Lateral surfaces

  • Basal surface

• Basement membrane consists of:

  • Basal lamina (from epithelial cells)

  • Reticular lamina (from connective tissue)

• Provides support and anchors epithelium to connective tissue.

Classification of Epithelial Tissue

• Covering and lining epithelia are classified by two criteria:

  • Shape of the apical cells (squamous, cuboidal, and columnar)

  • Number of cell layers (simple, stratified, pseudostratified)

Epithelial Tissue: Surface Epithelium (Descriptions, Locations, Functions)

• A. Simple Squamous Epithelium

  • Description: single layer of flat cells; nucleus centrally located; resembles tiled floor.

  • Location: lines cardiovascular and lymphatic systems (endothelium); serous membranes (mesothelium); air sacs of lungs; kidneys; cornea; tympanic membrane.

  • Function: site of filtration and diffusion; secretion in serous membranes.

• B. Simple Cuboidal Epithelium

  • Description: single layer of cube-shaped cells; round, centrally located nucleus.

  • Location: covers surface of ovary; anterior surface of lens capsule; pigmented retina; lines kidney tubules and small glands.

  • Function: secretion and absorption.

• C. Non-ciliated Simple Columnar Epithelium

  • Description: single layer of nonciliated columnar cells with oval nuclei near base; contains microvilli and goblet cells.

  • Location: lines GI tract, ducts of many glands, gallbladder.

  • Function: secretion and absorption; mucus lubricates digestive, respiratory, reproductive tracts; protects stomach from acidity.

• D. Ciliated Simple Columnar Epithelium

  • Description: single layer of ciliated columnar cells with goblet cells interspersed.

  • Location: lines some bronchioles, uterine tubes, uterus, paranasal sinuses, spinal canal, brain ventricles.

  • Function: cilia move mucus and particles toward the throat; goblet cells produce mucus; help oocyte transport.

• E. Non-ciliated Pseudostratified Columnar Epithelium

  • Description: appears multi-layered due to nuclei at various levels; all cells attach to basement membrane but not all reach apical surface; lacks cilia and goblet cells.

  • Location: lines epididymis, larger ducts of many glands, parts of male urethra.

  • Function: absorption and secretion.

• F. Ciliated Pseudostratified Columnar Epithelium

  • Description: similar to above but with cells that extend to surface and may bear goblet cells or cilia.

  • Location: lines most of the upper respiratory tract.

  • Function: secretes mucus; cilia move mucus out of passages.

• G. Stratified Squamous Epithelium

  • Description: two or more cell layers; apical cells are squamous; deeper layers may be cuboidal or columnar.

  • Process: basal cells divide and push upward; cells become dehydrated and die toward surface; keratinized and nonkeratinized varieties.

  • Location: Keratinized forms superficial skin; Nonkeratinized lines mouth, esophagus, part of epiglottis, part of pharynx and vagina; covers tongue.

  • Function: protection against abrasion, water loss, UV radiation, microbial invasion; first line of defense.

• H. Stratified Cuboidal Epithelium

  • Description: two or more layers with apical cells cube-shaped; relatively rare.

  • Location: ducts of adult sweat glands and esophageal glands; part of male urethra.

  • Function: protection; limited secretion and absorption.

• I. Stratified Columnar Epithelium

  • Description: basal layers are shortened irregular cells; apical layer contains columnar cells; uncommon.

  • Location: parts of urethra; large excretory ducts of glands (esophageal glands); small areas in anal mucosa; part of conjunctiva of eye.

  • Function: protection and secretion.

• J. Urothelium (Transitional Epithelium)

  • Description: variable appearance; relaxed state looks like stratified cuboidal; when stretched, appears more like stratified squamous.

  • Location: lines urinary bladder, ureters, portions of urethra.

  • Function: allows urinary organs to stretch and maintain protective lining as fluid volume changes.

  • Empty bladder vs full bladder demonstration shows tissue adapting to stretching.

Glandular Epithelium

• A gland is a single cell or a mass of epithelial cells adapted for secretion.

• Exocrine secretions enter ducts that empty onto a surface (skin or lumen).

• Endocrine secretions (hormones) enter interstitial fluid and diffuse into the bloodstream.

Glandular Epithelium (1 of 2)

• A. Endocrine Glands

  • Description: secretions (hormones) enter interstitial fluid then diffuse into bloodstream; no duct.

  • Locations: pituitary, pineal, thyroid, parathyroid, adrenal, pancreas (islets), ovaries, testes, thymus, etc.

  • Function: regulate metabolic and physiological activities to maintain homeostasis.

• B. Exocrine Glands

  • Description: secretions released into ducts that empty onto surfaces (skin or lumen of organs).

  • Locations: sweat, oil, earwax glands; digestive glands like salivary glands and pancreas.

  • Function: produce substances such as sweat, oil, saliva, digestive enzymes, etc.

Glandular Epithelium (2 of 2)

• Structural classification (unicellular vs multicellular):

  • Unicellular – goblet cells

  • Multicellular – sweat glands, oil glands, salivary glands, etc.

Multicellular Exocrine Glands

• Ducts and secretory portions can be arranged in various patterns:

  • Simple tubular, simple branched tubular, simple coiled tubular, simple tubular/acinar, simple branched acinar

  • Compound tubular, compound acinar, compound tubuloacinar

Functional Classification of Glandular Epithelium

• Secretion mechanisms include:

  • Merocrine secretion (exocytosis): example – salivary glands, most sweat glands

  • Apocrine secretion: involves a portion of the cell pinching off (apical portion) as secretion; example – mammary glands

  • Holocrine secretion: the entire cell disintegrates to become the secretory product; cell division replaces lost cells; example – sebaceous glands

Connective Tissue

• General features:

  • Consists of cells, extracellular matrix (ECM) with fibers

  • Connective tissue cells do not have free surfaces

  • Highly vascularized (blood vessels) and has a nerve supply (exceptions: tendons and cartilage)

Connective Tissue Cells, Fibers, and Ground Substance

• Key cell types include:

  • Fibroblasts: large flat cells that produce fibers and ground substance

  • Macrophages: phagocytize bacteria and debris

  • Mast cells: release histamine to dilate vessels during inflammation

  • Adipocytes: fat storage cells

  • Plasmocytes (plasma cells): secrete antibodies

  • Eosinophils and Neutrophils: white blood cells involved in parasitic defense and infection responses

• Fibers in the ECM:

  • Collagen fibers: strong and flexible, most abundant protein

  • Elastic fibers: stretchable; contain elastin and fibrillin; found in skin, vessels, lungs

  • Reticular fibers: form networks around fat cells, smooth muscle, and nerve cells. Very thin/ delicate

• Ground substance:

  • Gel-like material between cells and fibers

  • Made of water, hyaluronic acid, chondroitin sulfate, and other molecules

  • Supports cells and fibers, binds components, and mediates exchange between blood and cells

Connective Tissue Extracellular Matrix (ECM)

• Located in spaces between connective tissue cells

• Composed of fibers and ground substance

• Provides structural support and a medium for exchange between blood and cells

Classification of Connective Tissue (1 of 2)

• Embryonic: before birth

  • Mesenchyme

  • Mucous tissue

• Mature:

  • Loose connective tissue

  • Dense connective tissue

  • Cartilage

  • Bone

  • Blood

Connective Tissue: Anatomy Overview

• General study of tissue that supports, binds, and protects tissues and organs.

Mature Connective Tissue: Supporting Connective Tissue — Cartilage (1 of 3)

• A. Hyaline Cartilage

  • Description: resilient gel-like ground substance; bluish-white, shiny appearance; chondrocytes in lacunae; perichondrium present (except articular cartilage and growth plate regions).

  • Location: ends of long bones, anterior ribs, nose, parts of larynx/trachea/bronchi, embryonic skeleton.

  • Function: provides smooth joint surfaces, flexibility, and support; weakest cartilage type and can be fractured.

Cartilage: Fibrous Cartilage (2 of 3)

• Fibrous cartilage

  • Description: chondrocytes among thick bundles of collagen; lacks perichondrium.

  • Location: public symphysis, intervertebral discs, menisci of knee, tendons inserting into cartilage.

  • Function: provides support and joining structures; strong and rigid.

Cartilage: Elastic Cartilage (3 of 3)

• Elastic cartilage

  • Description: chondrocytes in a network of elastic fibers within ECM; perichondrium present.

  • Location: epiglottis, external ear (auricle), auditory (Eustachian) tubes.

  • Function: provides strength and elasticity; maintains shape of certain structures.

Mature Connective Tissue: Bone Tissue

• Description: two types of bone tissue:

  • Compact bone: osteons (Haversian systems) containing lamellae, lacunae, osteocytes, canaliculi, and central (Haversian) canals

  • Spongy bone: trabeculae with spaces filled by red bone marrow

• Location: throughout the skeleton; supports, protects, stores minerals, houses blood-forming tissue, acts as levers with muscles

Mature Connective Tissue: Liquid Connective Tissue — Blood

• Description: blood consists of plasma and formed elements: red blood cells (RBCs), white blood cells (WBCs), platelets

• Location: within blood vessels and heart chambers

• Function:

  • RBCs: transport oxygen and some carbon dioxide

  • WBCs: phagocytosis and immune responses

  • Platelets: essential for clotting

Membranes

• Membranes are flat sheets of pliable tissue that cover or line parts of the body

• Two types of membranes in the body:
  1) Epithelial membranes: mucous membranes, serous membranes, and cutaneous membranes (skin)
  2) Synovial membranes

Membranes (1 of 3)

• Epithelial membranes include:

  • Mucous membranes: epithelium plus lamina propria; line body cavities that open to the outside; goblet cells produce mucus.

  • Serous membranes: line closed cavities (do not open to outside) and cover organs; include serous fluid; mesothelium with areolar connective tissue.

  • Cutaneous membrane: skin (epidermis and dermis).

  • Synovial membranes: line joints with articular cavities; contain synoviocytes; secrete synovial fluid; lack epithelial layers.

Membranes (2 of 3)

• Visual examples: mucous membrane in intestinal tract; serous membrane in lungs (pleura).

• Key components: epithelium, lamina propria, serous fluid, mesothelium, areolar connective tissue, synoviocytes, synovial fluid, articular cavity.

Membranes (3 of 3)

• Skin (cutaneous membrane): epidermis and dermis; covers the body.

• Synovial membranes: lines joints containing an articular cavity and secrete synovial fluid to lubricate joints.

Muscular Tissue

• General characteristics: muscle tissue comprises fibers that provide motion, posture, and heat production.

• $3$ types of muscle tissue: skeletal, cardiac, and smooth.

Muscular Tissue: Anatomy Overview

• Overview of muscle tissue structure and its general roles in movement and homeostasis.

Muscular Tissue (1 of 3)

• A. Skeletal Muscle Tissue

  • Description: long, cylindrical, striated fibers.

  • Length varies; muscle fiber is multinucleated with nuclei at the periphery.

  • Voluntary control: contraction under conscious control.

  • Location: usually attached to bones by tendons.

  • Function: motion, posture, heat production, protection.

Muscular Tissue (2 of 3)

• B. Cardiac Muscle Tissue

  • Description: branched, striated fibers with usually one central nucleus.

  • Intercalated discs join fibers; desmosomes strengthen tissue; gap junctions enable rapid electrical conduction.

  • Involuntary control.

  • Location: heart wall.

  • Function: pumps blood throughout the body.

Muscular Tissue (3 of 3)

• C. Smooth Muscle Tissue

  • Description: nonstriated, spindle-shaped cells with a single central nucleus.

  • Gap junctions connect many fibers in some smooth muscle; some smooth muscle lacks gap junctions (e.g., iris).

  • Usually involuntary; can contract powerfully as many fibers act together.

  • Location: iris; walls of hollow organs (blood vessels, airways, stomach, intestines, gallbladder, urinary bladder, uterus).

  • Function: motion such as constriction/relaxation of vessels and airways, propulsion of contents through GI tract, contraction of bladder and gallbladder.

Nervous Tissue

• General characteristics: two kinds of cells – neurons and neuroglia.

• Neurons typically have a cell body, dendrites, and an axon; they can carry sensory or motor information and perform integrative functions.

• Neuroglia protect and support neurons.

Nervous Tissue: Anatomy Overview

• Nervous tissue is located in the nervous system and is specialized for excitability and rapid signal transmission.

Nervous Tissue (2 of 2)

• Description: consists of neurons (nerve cells) and neuroglia.

• Neurons: conduct nerve impulses and perform information processing.

• Neuroglia: support, protect, and nourish neurons.

Excitable Cells

• Neurons and muscle fibers are excitable: they respond to stimuli by producing electrical signals (action potentials).

• This electrical excitability arises from voltage-gated channels in the plasma membrane.

Tissue Repair: Restoring Homeostasis

• Tissue repair is the process of replacing worn-out, damaged, or dead cells.

• Epithelial cells are renewed by division of stem cells or undifferentiated cells.

• Not all connective tissue cells have repair capacity; muscle cells have limited repair; some nervous cells have limited repair.

• Fibrosis refers to scar tissue formation.

Aging and Tissues

• Younger bodies typically have:

  • Better nutritional state

  • Better blood supply to tissues

  • Faster metabolic rate

• Aging slows tissue repair and leads to stiffening and loss of elasticity in tissues.

Homeostatic Imbalances

• Disorders of epithelial tissues tend to be organ-specific (e.g., ulcers in the stomach).

• Disorders of connective tissue tend to be autoimmune (e.g., lupus).

• Disorders of muscular and nervous tissues will be discussed in later chapters.